The present invention relates to switches for connecting and disconnecting high voltage devices to electric power circuits and, more particularly, to a switch with external resistors and a high speed whip and drive mechanism for staged insertion of the resistors when connecting a capacitor bank to a circuit.
Electric power delivery systems such as those operated by electric utilities, large industrial facilities, military bases, and airports typically include a number of high voltage devices such as capacitor banks, voltage regulators, transformers, reclosers, surge arrestors, circuit breakers, and so forth. These devices are used in the operation of the system to maintain the quality of the electric power delivered at a power factor close to 1.0, to deliver the electric power at a certain voltage, to increase system reliability, and/or for other functions. Typically, each of these devices is connectable to the power circuit by a switch.
Conventional electric power switches have a male and a female contact that can be moved between a xe2x80x9cclosedxe2x80x9d position with the contacts in physical contact and an xe2x80x9copenxe2x80x9d position with the contacts physically separated. For an electric power line that carries a high voltage and/or high current, it is desirable to open and close the male and female contacts very quickly in order to avoid a pre-strike, in which the electric current arcs across a physical gap between the contacts. Pre-strikes impose high current spikes and serious voltage disturbances on the power line, and can also physically degrade the components of the switch, especially the contacts. These current spikes and voltage disturbances can also damage other pieces of equipment connected to the circuit.
Pre-strikes occur when the switch""s contacts are not yet in physical contact in the closing operation, but are still close enough to each other to permit electric current to arc through the air or other media between the contacts. When the contacts of a properly designed switch are fully separated in the open position, the distance between the contacts is sufficient to minimize pre-strikes. However, a pre-strike can occur as the contacts are moved through a xe2x80x9cclosing strokexe2x80x9d from the fully separate, open position to the fully connected, closed position. Likewise, an arc can occur across a gap between the contacts as the contacts are moved through an xe2x80x9copening strokexe2x80x9d from the closed position to the open position.
In order to minimize the occurrence of pre-strikes and their associated problems, xe2x80x9cinterrupterxe2x80x9d switches are often provided with high-speed mechanisms for closing the contacts either at voltage zero or after the voltage is minimized by a pre-insertion impedance which minimizes the closing transients. Such mechanisms are sometimes provided by spring-loaded mechanisms. Also, the contacts of interrupter switches are sometimes provided in a sealed housing with a dielectric gas, vacuum, or other media for quenching the arc. Additionally, interrupter switches are sometimes provided with a linkage connected to an actuator having an electric motor, fluid cylinder, or the like. Such linkages and actuators are designed for generating a large force to increase the velocity of the opening and closing strokes, operating the contacts of a three-phase switch simultaneously, and/or operating the switch remotely.
Typically, interrupter switches are designed to prevent restrikes when xe2x80x9copeningxe2x80x9d the switch under a load. However, when xe2x80x9cclosedxe2x80x9d switching a charged capacitor bank into the power circuit, conventional interrupter switches often are not able to prevent pre-strikes.
Charged capacitor banks are switched into the power circuit to correct the power factor during high-load periods, and later switched out of the circuit when the load drops. Capacitor banks store a charge, for example, 1 xe2x80x9cper unitxe2x80x9d (PU), and electric power systems operate at a system voltage, plus or minus 1 PU. Therefore, a conventional system-rated capacitor switch for connecting a 1 PU capacitor to the power circuit will be subjected to a 0 to 2 PU voltage surge when closing to connect the charged capacitor bank to the circuit, often resulting in intense high overvoltages. Additionally, capacitor banks are often connected and disconnected to the power circuit several times a day as the system load varies, resulting in multiple overvoltages each day.
Specialized capacitor switches have been developed in an effort to address this problem. One such type of capacitor switch has a series of sacrificial contacts that are designed to deteriorate over time as a result of current surges. However, these contacts must be regularly monitored and replaced as they deteriorate, thereby increasing the cost of using this type of switch. Because capacitor banks are often connected and disconnected to the power circuit more than once a day, the contacts must be monitored and replaced on a very strict basis. These switches do not prevent pre-strikes when connecting a charged capacitor bank to the power line, so the electric power system is still subjected to damaging current spikes and voltage disturbances. This is in part because these switches are generally based on conventional interrupter switch designs which prevent restrikes upon opening the switch, but for capacitor switching the potential for pre-strikes is greatest upon closing of the switch.
Another type of known capacitor switch includes a resistor in series with the capacitor when the capacitor is first connected into the circuit in order to reduce the current spikes. However, these devices tend to be unwieldy, bulky, and very difficult to time so that they are introduced into the circuit just as the capacitor is connected to reduce these inrush currents.
Accordingly, there is a need in the art for a capacitor switch for connecting a charged capacitor bank to an electric power circuit with controlled current spikes, that is easy to adjust for properly timing the switching operation, that is durable and reliable over thousands of operations, and that can be made and used at an affordable cost.
The present invention satisfies the aforementioned needs by providing a switch for gradually stepping a capacitor bank into an electric power circuit to compensate for power factor deviations. This is accomplished by providing two (or another number of resistors or other current limiting devices) and a conventional interrupter switch mechanism configured in series with the capacitor bank. Current flow is initiated in a staged sequence through the first resistor, then the second resistor, then the switch mechanism. The first resistor has a significantly higher resistance than the second resistor, which in turn has a significantly higher resistance than the switch mechanism. In this fashion, current flow from the charged capacitor bank is gradually stepped into the circuit, thereby significantly reducing electrical disturbances in the capacitor bank, the switch, and the circuit.
Additionally, the staged sequence of introducing the resistors and the switch mechanism is accomplished by the provision of a drive mechanism for operating an engagement arm to introduce the first and second resistors into the circuit, and an actuator for operating the switch mechanism. The drive mechanism and the actuator are operatively coupled together by a drive shaft or another linkage, with the drive mechanism, the actuator, and/or the drive shaft being readily adjustable to accomplish the desired timing of the sequence. The drive mechanism and the actuator are operable by the drive shaft to sequentially introduce the resistors into the circuit, then to introduce the switch mechanism and remove the resistors from the circuit so that the circuit has the full benefit of the capacitor bank for achieving power factor correction. Furthermore, the drive mechanism, the actuator, the switch mechanism, and the drive shaft can be provided by or made of relatively simple, readily available components, so that the switch is reliable, durable, and cost effective to implement in large quantities. For example, the switch mechanism can be provided by a conventional interrupter switch mechanism having a housing containing the contacts and a dielectric gas such as SF6, with the resistors disposed external of the housing.
Generally described, the switch includes a switch mechanism having a first contact and a second contact, an actuator mechanism coupled to the switch mechanism and operable to move the switch contacts between an open position and an closed position, a first resistor, a second resistor, an engagement arm such as a whip, a drive mechanism coupled to the engagement arm and operable to pivot the engagement arm between an open position and a closed position, and a drive shaft coupled to and operating the actuator mechanism and the drive mechanism. When the drive shaft rotates in a closing stroke, it operates the drive mechanism to pivot the engagement arm into contact with the first resistor then into contact with the second resistor, and operates to cause the actuator to close the switch mechanism contact just after the engagement arm contacts the second resistor.
In one aspect of the invention, the first and second resistors each have a contact adapted to receive the engagement arm, with each of the contacts positioned so that, when the engagement arm moves from the open to the closed position, the contact end of the first resistor receives the engagement arm before the contact end of the second resistor receives the engagement arm. Thus, the first resistor contact can have a length that is greater than a length of the second resistor contact.
In another aspect of the invention, the drive mechanism has a first hub that is coupled to the drive shaft and that moves between a first hub open position and a first hub closed position in response to rotation of the drive shaft, with the first hub having a latch release member. Also, the drive mechanism can have a second hub that is spring-biased to move between a second hub open position and a second hub closed position in response to rotation of the first hub, with the second hub having a catch member and where the engagement arm is coupled to the second hub. Additionally, the drive mechanism can have a movable latch member that is biased towards an engaged position where the latch member contacts the catch member and prevents movement of the second hub from the second hub open position to the closed position. The latch release member can be positioned so that, when the first hub is moved from the first hub open position toward the closed position, the latch release member contacts and moves the latch member away from the second hub into a disengaged position, thereby permitting the second hub and the engagement arm to move from the open position to the closed position at a high velocity under the force of the spring.
In yet another aspect of the invention, the latch member has an adjustable closing latch member that is positioned so that, when the first hub is moved from the first hub open position toward the closed position, the latch release member contacts the adjustable closing latch member. Also, the second hub can have an adjustable stop member positioned thereon so that, when the first hub moves from the first hub closed position toward the open position, the latch release member contacts the stop member and causes the second hub to move from the second hub closed position to open position. Additionally the latch member can have an opening latch surface defined thereon so that, when the second hub moves from the second hub closed position to the open position in response to movement of the first hub from the first hub closed position to the open position, the catch member contacts the opening latch surface and moves the latch member from the latch engaged position to the disengaged position.
In a further aspect of the invention, there are provided three of the switches forming a three-pole switch for use in a three-phase electric power circuit. Additionally, a three-pole operator mechanism with an operator can be connected to the three-pole switch, for remote operation of the three-pole switch.
Another aspect of the invention is that the switch can be configured with the resistors and the interrupter switch mechanism in series, with the drive mechanism configured for generating a whip action during the opening stroke. In this manner, the switch can be used to split the voltage during and achieve a smoother opening of the switch.
In yet another aspect of the invention, a separate whip or other engagement arm can be provided for each resistor and contact. In this manner, there is provided greater flexibility and reliability of the switch.
In still a further aspect of the invention, there is provided a method for switching an electrical device into an electric power circuit. The method can include the steps of providing a switch mechanism, a first resistor, and a second resistor, initiating a current flow through the first resistor, initiating a current flow through the second resistor and limiting the current flow through the first resistor, and initiating a current flow through the switch mechanism and limiting the current flow through the first and second resistors. Also, the steps of initiating a current flow through the first and second resistors can include providing an engagement arm pivotally coupled to the switch mechanism and pivoting the engagement arm from an open position separated from the first and second resistors to a closed position in contact with the first and second resistors so that the engagement arm contacts the first resistor before the engagement arm contacts the second resistor.
Additionally, the step of pivoting the engagement arm from the open position to the closed position can include providing at least one drive mechanism coupled to the engagement arm, providing a rotary drive shaft operatively coupled to the drive mechanism, rotating the drive shaft, preventing pivoting of the engagement arm and generating a spring-loaded force urging the engagement arm to pivot from the open position to the closed position, releasing the engagement arm, and pivoting the engagement arm in response to the spring force. Furthermore, the step of initiating a current flow through the switch mechanism can include providing an actuator mechanism operatively coupled to the switch mechanism and coupled to the rotary drive shaft, and actuating the actuator, in response to rotation of the drive shaft, to close the contacts of the switch mechanism to initiate current flow through the switch mechanism and limit the current flow through the first and second resistors.